Tuning fork piezoelectric resonators are known in which the electrical excitation field is created in a direction parallel to one of their faces by electrodes, some of which are arranged on said face. Such a tuning fork resonator 1, shown in FIG. 1, includes a base 2 and two vibrating arms 3a and 3b extending from the base. With a view to reducing energy consumption owing to excitation by a more homogenous electrical field, there is known, particularly from FR Pat. No. 2 467 487, which is incorporated herein by reference, to provide at least one groove 4a and 4b on the bottom and/or top faces of each of the arms, such that the arms generally have an H-shaped cross-sectional form, as is visible in the cross-section A-A of FIG. 1.
Although of small dimensions, of the order of a millimeter, such piezoelectric generators are still too bulky in light of the ever increasing demand for miniaturisation of electronic equipment, such as portable telephones or watches.
One solution recommended in the prior art, shown in FIG. 2, and described in more detail in U.S. Pat. No. 6,587,009 consists in providing notches 5a and 5b on the flanks of base 2 of the piezoelectric generator. The main purpose of these notches 5a and 5b is to mechanically uncouple zone 6 of base 2, used to fix resonator 1 in its case, from vibrating arms 3a and 3b. Notches 5a and 5b enable, more specifically, the vibrations, which generate a movement in the zone of shoulders 7a and 7b, to be uncoupled. This mechanical uncoupling between base fixing zone 6 and vibrating arms 3a and 3b enables the size ratio between base 2 and arms 3a and 3b to be reduced and thus the total size of resonator 1 to be reduced without thereby altering the operating mode.
However, such a solutions has certain drawbacks. An analysis of this prior art solution has demonstrated that these notches 5a and 5b do not enable the tuning fork to be uncoupled from thermal stress generated by the lack of thermal tuning of the thermal expansion coefficients, thus resulting in the propagation of a static mechanical stress in a median zone 8 of base 2 located between the two arms 3a and 3b. Moreover, dynamic elastic stress is maximal in this median zone 8. The results of this combination of static mechanical stress varying with temperature and the dynamic elastic stress is an alteration in the features of resonator 1.
One solution for improving the resistance of the resonator to these types of stress is the use of a flexible adhesive agent, like for example conductive silicon adhesives, for mounting the resonator in its case. Nonetheless, such flexible adhesive agents exhibit problems of adherence and resistance to shocks, such a solution is thus undesirable.
Moreover, notches 5a and 5b weaken the mechanical structure of resonator 1, which raises a problem in the event of shocks, and more specifically in the event of lateral shocks. Indeed, when there is a lateral shock, the forces exerted on the whole of resonator 1, i.e. on arms 3a and 3b and on base 2, result in maximum stress at the point of intersection between each notch 5a and 5b and base fixing zone 6.
It is one of the main objects of the invention to overcome the aforementioned drawbacks by making a piezoelectric resonator having, on the one hand, a reduced size as well as an equally reduced energy consumption, and on the other hand, good resistance to the various stresses that the resonator may undergo, as well as good shock resistance.